In the drilling and production of oil and gas wells, a well bore is drilled by means of a plurality of drill pipes provided in sufficient numbers to assemble a rotatable drill string sufficient to drill the required depth. The rotatable drill string is terminated by a drill bit and typically provided with stand-off stabiliser parts periodically throughout the length of the drill string. The drill string is rotated to remove formation ahead of the drill bit, to drill out and thus form a wellbore, and to increase the depth of the well. Drilling mud or other fluid is circulated through the drill string to cool, lubricate and clear the drill bit of cuttings, and to displace the resulting drill cuttings from the bottom of the well to the surface, via an annulus formed between the drill string and the wall of the wellbore.
Periodically, the drill bit is removed from the wellbore and a casing comprising lengths of tubular casing sections coupled together end-to-end is run into the drilled wellbore and cemented in place. A smaller dimension drill bit is then inserted through the cased wellbore, to drill through the formation below the cased portion, to thereby extend the depth of the well. A smaller diameter casing is then installed in the extended portion of the wellbore and also cemented in place. If required, a liner comprising similar tubular sections coupled together end-to-end may be installed in the well, coupled to and extending from the final casing section. Once the desired full depth has been achieved, the drill string is removed from the well and then a work string is run-in to clean the well. Once the well has been cleaned out, the walls of the tubular members forming the casing/liner are free of debris so that when screens, packers, gravel pack assemblies, liner hangers or other completion equipment is inserted into the well, an efficient seal can be achieved between these devices and the casing/liner wall.
The step of cleaning the wellbore is usually achieved by inserting a work string containing dedicated well clean-up or cleaning tools. Typical well cleaning tools known for use in this environment include scrapers, wipers and/or brushes which are held against the internal wall of the casing/liner, to clean away debris as the tool is run-in and then pulled out of the wellbore. While this process is effective in cleaning the wellbore, it adds a significant amount of time to the job of preparing the well for production, since a separate well clean-up string requires to be run in the bore after the drill string has been removed. Thus at least two trips are required, a drill string trip and a work string trip.
Additionally, the formation in the wellbore is left exposed during the intervening period whilst the drill string is pulled out the hole and during the clean-up operation, which is disadvantageous because it is known that problems may arise in leaving a formation exposed between drilling and completion of a well.
One known type of cleaning apparatus is disclosed in UK Patent Publication No. 2 327 963 (Appleton et al). GB 2 327 963 describes a work string combining a packer with a scraper. The scraper is used to clean the casing ahead of the packer so that the packer can be set against a debris free casing. While this method removes the requirement of running a separate clean-up string before the packer is inserted, such a string is limited in that the scraper can only clean a fixed distance ahead of the packer and, as a result, only a portion of the casing is cleaned. Also, any debris dislodged by the cleaning operation is pushed into the wellbore.
Another cleaning tool is described in WO 2006/016102 (Fitzgerald et al) which is intended for cleaning casing. The tool proposed therein has retracted cleaning scrapers for run in upon a drill string, which are extended under spring-loading only after drilling is stopped. When the bore hole has been extended sufficiently below the casing by drilling, the drill string is pulled out to locate the cleaning scraper within the casing where a liner hanger is to be located. A cleaning operation is then conducted upon the casing as the drill string is rotated and reciprocated. Having the cleaning scrapers retracted during drilling, allows the casing to be cleaned on the return trip of the drill string, and spring loading allows the cleaning scrapers to be extended for cleaning the casing.
It is found that when operating near bit tools in the open (no casing or liner) borehole, ingress of debris and formation particulates leads to problems with the tools.
It is amongst the objects of at least one embodiment of the present invention to obviate or mitigate at least one of the foregoing disadvantages.
It is also amongst the objects of at least one embodiment of the present invention to provide a method of drilling a well wherein the well may be selectively cleaned at any depth whilst the drill string is rotated in the wellbore.